1. Field of the Invention
The present invention relates to a photoelectric converter used with a bar code reader, a facsimile apparatus, a digital copier or the like and more particularly to a photoelectric converter of the type having an auxiliary electrode or of the thin film transistor (hereinafter called TFT) type. A gate electrode is provided on a semiconductor layer with an insulating layer interposed therebetween.
2. Related Backgound Art
Recently, as electronic office machines such as facsimiles or digital copying machines become popular, the demand for a small type of inexpensive image input device has increased. Coplanar photoelectric converter (photosensor) which use a-Si, CdS-CdSe or the like as the photoconductors to utilize a photoelectric conversion effect have the advantage that they can directly contact an original document, and requiring no focusing system. Furthermore, they have a short travelling distance of the focusing system. In order to stabilize the sensor characteristics, field effect type photosensors are provided which have an insulating layer and an auxiliary electrode above or below a semiconductor layer among the coplanar type photosensors.
FIGS. 1(a) and (b) are schematics of the photoelectric conversion unit of the photosensor. In FIG. 1(a), an auxiliary electrode and insulating layer are formed on an insulating substrate 1 of glass, ceramic or the like The auxiliary electrode 2 and the insulating layer 3 on which is formed a semiconductor layer 4 are made of CdS.Se, a-Si: H or the like. A pair of main electrodes 6 and 7 are formed on the substrate through the intermediary of a doped semiconductor layer 5 for ohmic contact. A photoreception window 8 is formed between the main electrodes.
The photosensor having a structure shown in FIG. 1(b) has the auxiliary electrode 2 provided over the main electrodes 6 and 7. The substrate 1 is made of a transparent material and receives light from the side of the substrate 1. A part having the same function as the corresponding one of the photosensor of FIG. 1(a) is given the same number.
In the photosensor having the above structure, in order to get a large ratio of the output current to the dark current in response to incident light, an appropriate bias must be applied to the auxiliary electrode 2 in accordance with the kind of majority carriers of a current flowing through the semiconductor layer 4 to operate the photosensor. Namely, a negative bias must be applied when the majority carriers are electrons while a positive bias must be applied when the majority carriers are holes. Such operation will reduce the output current, so that a capacitor which stores the output current must be provided in the circuit which reads the signal from the photosensor. Correspondingly, switching elements such as a TFT which transfers as a signal the electric charges stored in the capacitor and a TFT which discharges the remaining charges after the transfer operation, and a matrix circuit which connects these switching elements are required.
FIG. 2 is a schematic equivalent circuit diagram of a plurality of line-sensor type photoelectric conversion elements disposed in an array. In FIG. 2, reference characters Rs1, Rs2, . . . Rsn denote photosensors having an auxiliary electrode; Cs1, Cs2 . . . Csn storage capacitors; Tt1, Tt2 ... Ttn TFTs which transfer electric charges stored in the capacitors Cs1-Csn; Tr1, Tr2 . . . Trn discharge TFTx which discharge electric charges remaining in the capacitors Cs1-Csn after the transfer operation. Reference characters Lg1-Lgn denote leads connected to the control electrodes or gates of the transfer TFTs Tt1-Ttn and the control electrodes or gates of the discharge TFTs Tr1-Trn. Reference characters Vg1-Vgn denote switching voltages applied to the corresponding leads Lg1-Lgn. Reference character LD denotes a lead connected to the main electrodes of the photosensors Rs1-Rsn. Reference character VD denotes a voltage applied to LD. Reference character LR denotes a lead connected to the storage capacitors Cs1-Csn. Reference character VR denotes reference voltage for storage capacitors Cs1-Csn. If the storage capacitors Cs1-Csn are discharged via discharge TFTs Tr1-Trn, the voltages across the capacitors Cs1-Csn will become VR.
In the photosensor, if a steady state bias is applied to the auxiliary electrode in accordance with the kind of majority carriers in the photosensor, namely, if a negative bias is applied when the majority carriers are electrons and if a positive bias is applied when the majority carriers are holes, the minority carriers will collect in the vicinity of the auxiliary electrode in the semiconductor layer and the majority carriers will collect in that portion of the semiconductor layer opposite the auxiliary electrode. In such condition, the majority and minority carriers are not re-combined smoothly, so that even if irradiation light to the photosensor may be cut off, the remaining photocurrent will flow as long as the minority carriers continue to exist. As a result, the optical response speed and hence the S/N ratio of the photosensor are lowered.
In an attempt to cope with this, for example, as shown in FIG. 3, if a bias voltage V1 of a predetermined level, negative when the majority carriers are electrons and positive when the majority carriers are holes, is applied in advance to the auxiliary electrode when the photosensor is to be read and if a pulse voltage of V2 opposite in polarity to the bias voltage is applied to the auxiliary electrode during a non-reading interval provided immediately before reading, a rise in the optical response would be improved to provide a large ratio of the output current to the dark current in response to incident light.
However, if such operation is tried on, for example, an image reading apparatus including a multiplicity of one-dimensionally arranged such photosensors, the respective sensor bits are read in a time series, so that the timings of applying the respective pulse voltages are shifted bit by bit. Thus, if voltages applied to the auxiliary electrodes of the photosensors are controlled in a circuit separated from the circuit in which the gate voltages of the transfer and discharge TFTs are controlled, a matrix circuit for control of the auxiliary electrodes of the photosensors and a matrix circuit for control of gate electrodes of the transfer TFTs would be required. Thus the entire circuit would become complicated. In addition, the timings of applying pulses, pulse widths, pulse magnitudes, etc., must be determined individually, so that the driving would become complicated.
Next, an example of the structure of a photoelectric conversion section of a photosensor of TFT type is shown in FIGS. 4 and 5. FIG. 4 is a plan view of the photoelectric conversion section, and FIG. 5 is a cross section taken along line X--X' of FIG. 4. In the Figures, the photoelectric conversion section comprises a substrate 401 made of glass for example, a gate electrode 402, an insulating layer 403, a photoconductive semiconductor layer 404, source and drain electrodes 406 and 407, an n.sup.+ layer for ohmic contact between the semiconductor layer 404 and the source and drain electrodes 406 and 407.
Dark current of a TFT type photosensor can be controlled by applying a bias voltage to the gate electrode to suppress the effect of the surface of the insulating layer. Thus, a favorable light quantity dependence characteristic (hereinafter called .lambda.) of a photoelectric conversion output is obtained which is as near as 1. The reproductiveness is also satisfactory with little manufacture deviation within a lot and among lots.
These characteristics lead to advantageous effect under static (DC) drive conditions. However, under dynamic drive conditions commonly employed for image sensors or the like, i.e., under a charge storage mode, there may arise a problem which is described hereinafter.
FIG. 6 is a circuit diagram of a readout circuit using a TFT type photosensor under a charge storage mode. Connected to a drain electrode is a sensor power supply V.sub.S and to a gate electrode a bias power supply V.sub.B. Connected to a source electrode is a storage capacitor C. The charge stored in the storage capacitor is discharged to a load resistor R.sub.L upon activation of a transfer switch SW.
Operating waveforms of the circuit are shown in FIG. 7. The transfer switch SW repeatedly turned on and off at the period of a storage time T.sub.S. Namely, while the transfer switch SW. is being turned off, photocurrent i.sub.S of the photosensor is charged in the storage capacitor. Whereas while the transfer switch SW is made turned on, the charge stored in the storage capacitor C is caused to be discharged to the load resistor R.sub.L which in turn is read as an output of the photosensor.
Voltage V.sub.C appearing across the storage capacitor C can be represented in terms of an integrated value of i.sub.S under the condition of V.sub.S &gt;&gt;V.sub.C :V.sub.C =.intg..sub.o.sup.t i.sub.S dt=i.sub.S xt . Thus, the voltage V.sub.C increases substantially linearly with respect to time t. This increase in voltage is shown by a broken line in FIG. 7.
However, in practice, when the photosensor in the circuit of FIG. 6 is driven a distored waveform of the voltage V.sub.C as shown by a solid line in FIG. 7 is obtained. The reason for this is that the voltage V.sub.C quickly changes to zero when the transfer switch turns on and a gate bias voltage .DELTA. Vgs is made relatively small so that transient current i.sub.a (hatched portion in FIG. 7) flows through the source and drain. This transient current adversely effects the light dependence characteristic of an output from a photoelectric converter constructed of the above circuit, to thereby lower the gamma .lambda. value to 0.4 to 0.5as shown in FIG. 8, which is considerably small as compared to the gamma .lambda. value 1 calculated based on the static drive conditions, and thereby deteriorates an S/N ratio.